Color former for pressure sensitive recording paper and process for producing same

ABSTRACT

A color former for pressure sensitive recording paper which comprises a member selected from the group consisting of acidtreated dioctahedral montmorillonite clay minerals and mixtures of said minerals with natural dioctahedral montmorillonite clay minerals and a process for producing the same.

United States Patent 72] Inventors Yujiro Sugahara Tokyo; KaichiroMiyazawa, Tsuruoka-shi; Tadahisa Nakazawa, Niigata-ken; Masahlro Maeno,Niigata-ken, all of Japan [2]] Appl. No. 775,126 [22] Filed Nov. 12,1968 [45] Patented Nov. 23, 1971 [73] Assignee Kabushiki Kalsha MizusawaKagaku Kogyo l-llgashi-ku, Osaka, Japan [54] COLOR FORMER FOR PRESSURESENSITIVE RECORDING PAPER AND PROCESS FOR [50] Field of Search 106/72,288 B; 252/450 [56] References Cited UNITED STATES PATENTS 2,903,4349/1959 Gloss 252/450 3,213,037 10/1965 l-lodgkiss 252/450 3,369,9932/1968 Mills et al. 252/450 Primary Examiner-James E. PoerAttorney-Sherman and Shalloway ABSTRACT: A color former for pressuresensitive recording paper which comprises a member selected from thegroup consisting of acid-treated dioctahedral montmorillonite clayminerals and mixtures of said minerals with natural dioctahedralmontmorillonite clay minerals and a process for producing the same.

COLOR FORMER FOR PRESSURE SENSITIVE RECORDING PAPER AND PROCESS FORPRODUCING SAME This invention relates to a color former whichdemonstrates pronounced color development effects when used in makingmanifold record paper, i.e. the pressure sensitive recording paper,which can reproduce copies by handwriting, printing or typing withoutthe necessity of the conventional carbon paper, The invention alsorelates to a process for producing such a color former.

The pressure sensitive recording papers, with a few exception in thecase of special papers, are in all cases those in which the colordevelopment reaction ascribable to the transfer of electrons between thecolorless compound of organic coloring matter having electron donatingproperty and a color former, the electron acceptor. (US. Pat. No.2,548,366).

As the colorless compound of organic coloring matter, the color reactionsubstance, two classes of coloring matter each of which exhibitdifferent behaviors of coloration are used cojointly. One of them isthat which, as in the case, for example, of the triphenyl methanecoloring matter, develops color intensity immediately upon contacting asolid acid, but which has a tendency to fade easily (primary colordevelopment coloring matter). The second coloring matter used is onewhich does not immediately develop color upon contacting a solid acidbut develops its color completely after several days have elapsed andexhibits adequate fastness to sunlight. As such a coloring matter, forexample, the acyl leucomethylene blue coloring matter are used(secondary color development coloring matter).

On the other hand, as the color former, the electron acceptor, the solidacids are generally used. In the past, known are such, for example, askaolin, bentonite, attapulgite, aluminum sulfate, natural zeolite,silica gel, feldspar, pyrophyllite, halloysite, magnesium trisilicate,zinc sulfate, zinc sulfide, calcium fluoride, calcium citrate as well asthe organic acids as tannic acid and benzoic acid.

The pressure sensitive recording paper using these color developmentcoloring matter and color formers is made up of two classes of papers:one the transfer sheet (referred to as the coated back of CB), a paperwhich has been coated with the coloring matter in solution in oil andencapsulated by such as gelatin, gum arabic or synthetic resin the sizeof which capsules is several microns in diameter, and the other thereceiving sheet (referred to as the coated front or CF a paper coatedwith the color former. Now, when the foregoing two papers are superposefacing each other and pressure is applied with either a steel pen ortypewriter, the capsules of that portion to which the pressure has beenapplied rupture and the oil and the colorless coloring matter come intocontact with the color former to develop color and thus impress thatportion with a mark. On the other hand, when three or more copies arerequired, an one or more intermediate sheets which we generally referredto as a coated front and back back sheet (or CFB) i.e. one which has thefront coated with the color development coloring matter and the backcoated with the color former, are used interleaved between the transfersheet and the receiving sheet.

According to the disclosures made heretofore, it can be seen that in allcases, the researches concerning the pressure sensitive recording paperhave laid their emphasis on the process of synthesizing the organiccoloring matter and capsulation thereof and practically no studies havebeen made regarding the color former of the pressure sensitive recordingpaper. Thus, in the present state of the art the practice is to useattapulgite, a kind of naturally obtained clay, in its as-obtainedstate.

However, the conventional color formers such as indicated above wereeither those in which notwithstanding their good color developmenteffect relative to the primary color development coloring matter theircolor development effect relative to the secondary color developmentcoloring matter was poor or those in which notwithstanding their goodcolor development efi'ect relative to the secondary color developmentcoloring matter their color development effect relative to the primarycolor development coloring matter was poor. Thus, there has not beenfound to date a color former which demonstrates excellent colordevelopment effects relative to the primary as well as secondary colordevelopment coloring matter.

It is therefore a primary object of the present invention to provide acolor former for use with the pressure sensitive recording paper, whichdemonstrates excellent color development effects relative to the primaryas well as secondary color development coloring matter. Another objectis to provide a process by which the foregoing color former is produced.A still another object is to provide a color former for use with thepressure sensitive recording paper, which not only excels in its colordevelopment effects relative to the primary as well as secondary colordevelopment coloring matter but also causes fewer smudges. A furtherobject is to provide a process for producing such a color former.Smudge, as here used, refers to a nonintended color developmentphenomenon (soiling) which occurs, for example, during preservation,carrying, handling, etc. An additional object is to provide a colorformer for pressure sensitive recording paper in which the exfolidationphenomenon after its application to the paper is less.

Other objects and advantages of the present invention will becomeapparent from the following description.

The foregoing objects and advantages are achieved according to thepresent invention by a color former for pressure sensitive recordingpaper which comprises a dioctahedral montmorillonite clay mineral and/orthe acid-treated products thereof the specific surface area of which isat least mF/g. of the total particles at least 75 percent by weighthaving a particle diameter 10 microns or less, and not more than 45percent by weight being those of diameter one micron or less, saidmineral and/or its acid-treated products having a secondary colordevelopment property, K of at least 1.40, preferably at least 1.60, thevalue of K being represented by the formula wherein R and R arereflectances of light having wavelengths 430 my. and 555 mp.,respectively when said mineral and/or its acid-treated products aredeveloped by benzoyl leucomethylene blue.

As a result of our extensive researches concerning the color former forpressure sensitive recording paper, we found that the followinginteresting facts.

According to our investigations, it was found that the color developmenteffects of the color former in the secondary color development coloringmatter, e.g. acyl leucomethylene blue, was controlled to a great extentby the inherent properties of the natural solid acid, and that thoughthe color development property of the natural solid acid relative to thesecondary color development coloring matter could be improved somewhatby such chemical treatments as, say, acid, alkali, oxidation andreducing treatments, a substantial improvement of the color developmentproperty could not be attained. Further, as the solid acids, which arethe color former, the natural clay minerals, such as hereinbeforeindicated, e.g. kaolin, bentonite, attapulgite and natural zeolite, areknown, but actually attapulgite is principally used.

However, it was found according to our investigations that the secondarycolor development property of these natural clay minerals was veryirregular, there being a marked difference in the color developmentproperty even among those of the same class depending upon such as theirlocale of production or their position of burial within the samedeposit.

Hence, when the natural clay minerals generally referred to as, forexample, attapulgite or bentonite, are used nonselectively, little, ifany, color development property is noted n some instances or the extentof the color development is not adequate in some instance, and thus auniform secondary color development effect is not demonstrated. Inaddition, these natural clay minerals have the shortcoming that theirprimary color development efiect is on the whole uniformly poor.

We however found according to our studies that of the natural clayminerals the dioctahedral montrnorillonite clay mineral had a uniqueproperty in that a certain class of the montmorillonite clay mineralshad as its inherent property an excellent color development propertyrelative to the secondary color development coloring matter (secondarycolor development property). It was also found that it differed from theother clay minerals in that its color development property relative tothe primary color development color-ing matter (primary colordevelopment property) could be enhanced remarkably by an acid treatment.Further, it was continued by our studies that a practically proportionalrelationship holds between the primary color developments property ofthe hereinbefore indicated clay minerals and their specific surfacearea; and that the specific surface area of dioctahedral montmorilloniteclay minerals adopted by the present invention is generally increased byits acid treatment though there is a difference in degree depending uponits class and that an enhancement of the primary color developmentproperty takes place in concomitance with this increase in specificsurface area.

We found that a color former for pressure sensitive recording paper usewhich excels in both in its primary and secondary color developmentproperty could be obtained by first choosing from among the dioctahedralmontmorillonite clay minerals those which excel in secondary colordevelopment property and thereafter subjecting these chosenmontmorillonite clay minerals to an acid treatment until the desiredspecific surface area is obtained.

Further, we found according to our investigations that the colordevelopment property of the aforesaid montmorillonite clay mineralscould be readily determined by measuring in accordance with themeasurement method given below the reflectances R and R of light havingwavelengths 430 my. and 550 mp. when said minerals are developed bybenzoyl leucomethylene blue of the formula and comparing the value of Kcalculated as follows:

'primary color development property,

thereof Five grams of this powder is then placed in a weighing bottleand is dried in a 1 10 C. constant temperature dryer for 1 hour followedby allowing it to cool in a desiccator.

The particle diameter of the color former is measured by means of theAndreasen sedimentation pipet. For particulars, reference shall be madeto the Encyclopedia of Chemical Technol gy (RE. Kirk, D. Othmer), Vol.12, p. 490 (1954). The particle diameters used herein have all been madeby this method of measurement.

2. Preparation of the color development coloring matter solutron.

Five grams of benzoyl leucomethylene blue are dissolved in grams of g.p.benzene.

3. Color development conditions.

Two grams of the foregoing dried specimen are weighed onto a watch glass8 cm. in diameter and spread out thinly. Four cc. of the aforesaidbenzoyl leucomethylene blue solution are then dropped in such a manneras to completely wet the whole specimen, after which stirring is carriedout with a spatula to ensure that the whole becomes homogeneous. lfcomplete wetting of the specimen cannot be accomplished by 4 ml. of thesolution owing to the greatness of the oil absorption value of thespecimen, the foregoing operation is carried out after first addingbenzene to the coloring matter solution in a necessary amount. This isfollowed by allowing the specimen to stand for 24 hours in a room oftemperature l5-20 C. into which the direct range of sunlight do notshine. During this time specimen is mixed two or three times with aspatula to ensure that the whole becomes homogeneous. The so obtainedspecimen is used for measuring the degree of color development.

4. Method of measuring the degree of color development.

The degree of color development of the specimen whose color has beendeveloped in the foregoing manner is measured with a spectrophotometer.The specimen is packed in a powder cell of quartz and its reflectancesof lights having wavelengths of 430 and 550 mp. are measured. Thereflectance is expressed in percent using as the basis percent for thereflectance of an alpha-alumina shaped product. 5. Method of indicatingthe secondary color development property.

When the reflectances of the lights of 430 my and 550 mu are designatedrespectively R and R the secondary color development property is definedby the following equation:

Thus, the greater the value of H the better the secondary colordevelopment property. While as the method of accurately indicatingcolor, the methods of indication of the Commission InternationaldEclar-iage or International Commission on Illumination are used. Thedeterminations of color development by means of the value of K,, asdefined herein above and the results of determinations by means of thenaked eye were found to be in very good agreement in our experiments.

Next, for clarifying the fact that the specific surface area secondarycolor development property and decolorizing property of the natural clayminerals differ greatly depending upon such conditions as their classlocale of production and position of production is the same locale andthat there is a change over a broad range in these properties after theacid treatment of the minerals, these properties of the various classesof natural clay minerals are shown in table I,

TABLE I Property of untreated specimen Acid treatment Specific SecondaryDecoloriz- Specific Prim surface Primary color deg surface color deve Siccimen Class of the clay Name of the clay Locale in which area colordevelvelopment ability, area opment umber mineral mineral clay produced(rnJ/g.) opment (K (K2) percent (mi/g.) (K 5 Attapulgitefittfiifihlltglte llorid a (X -8 2k.) 32 2. 89 1.68 34.4 160 2.65 ao eeorg a 2. 6 1.17 39. 5 40 2.38 3 iKmun mineral '{Halloysite iigate 72 2.20 1. 35 29. 3 2g 4 Mica clay mineral... Sericite Niigata. 24 2. 71 1.20 39. 5 24 3.03 ale mineral Talc. kayama 2.02 1. 18 0 10 2.08 Bentomlte(I) Okayama 1.74 1.19 0 1.78 Bentomite (II) Tsugawa 60 2.40 1. 70 6.8136 2. 50

(Niigata). Bentomite (III) Gunma 72 2.63 1.84 6.8 100 2.22 Sub-hentonite(I) MESlSSSlRp; 104 2. 40 1. 80 5. 7 280 3.04 Sub-hentonlte (II) Utah U.S:A.) 72 2.48 1. a0 1.1 232 2.96 Dioctahedralmont- Japanese-acid clay(I)- N akajo (Niigat 104 2.45 2.15 9. 1 200 3. 15 12 morillonite Japanese-acid clay do 96 2. 48 1.63 18.2 240 2. 70 1a {firgmese-acid clayShibata (Nligata) 12s a. 10 1. 29 42. 1 160 a. 43 14 Japanese-acid clayTsuruoka 104 2. 42 2.15 33. 0 350 3. 21

IV (Yamagata). 15 Jagpese-acid clay .do 136 2.19 1. 39 18. 2 144 2. 35

Property of acid-treated specimen (A) Property of acld-treated specimen(B) Secondary Decolor- Acid Specific Primary Secondary Decolor- Acidcolor deizing treatsurface color decolor deizing treat- Suitability foruse as color former Specimen velopment ability, ment acid velopmeutvelopment ablllty, ment for pressure sensitive recording Number (K2)Percent time (mfi/g.) (K1) (K1) percent time paper 1. 65 52. 1 1 160 2.65 1. 65 47.0 2 Satisfactory (primary color develop ment efiectinferior) 1. 21 47. 8 3 72 2. 38 1. 22 54. 2 6 Poor. 1. 33 74. 4 3 1902. 33 1. 32 79. 7 6 Poor (decolorizlng property very good). 1. 20 39. 61 32 3. 11 1. 20 42. 5 5 P001". 1.16 0 1 10 2.19 1.14 2.0 4 D0. 1.18 0 l1.90 1. 15 0 2 D0. 1. 80 72. 0 1 185 3. 10 1. 82 73. 2 3 Excellent. 1.82 66.2 1 120 2. 23 1. 71 70. 2 3 Poor. 2. 22 87. 1 5 350 3. 41 2. 2675.7 9 Excellent (grade become acceptable as a result of acidtreatment). 1.30 73.3 2 328 3.03 1. 25 80.3 4 Poor (decolorizingproperty very good). 1.89 75. 3 2 280 3. 47 1. 72 80. 7 5 Excellent(grade become acceptable as a result of acid treatment). 1. 98 68.0 3340 3. 03 1.89 78. 2 7 Do. 1. 64. 7 4 168 3. 69 1. 32 70. 4 8 Poor. 2.29 70. 8 3 400 3. 42 1. 57 73. 8 6 Excellent (grade become acceptable asa result of acid treatment). 1. 35 68.7 2 295 3. 20 1.33 78. 9 4 Poor(decolorizing property good).

Testing procedures. 45 76.5 Grams each of the clay on a dry basis wereweighed The several tests indicated in table I were conducted in theinto eight 500-ml. conical beakers. After adding 200 ml. of 34 followingmanner: weight percent sulfuric acid to each beaker, they are heated inl Preparation of the specimen.

After the dried color former has been fully comminuted in a mortar orpot mill, it is winnowed and prepared such that a least 85 percent byweight of the total particles are those having a particle diameter 10microns or less and not more than 35 percent by weight are those of aparticle diameter one micron or less. Five grams of this powder are thenweighed into a weighing bottle, dried for 1 hour in a 1 [0 C. constanttemperature dryer and thereafter allowed to cool in a desiccator.

2. Acid treatment conditions. Specimens Nos. l-8.

Fifty grams each of the clay on a dry basis are weighed into six 500-ml.conical beakers. After adding 300 ml. of 16.2 weight percenthydrochloric acid to each beaker, they are heated in a 85 C. water bath.After the passage of each hour one of the beakers is taken out by turnsfrom the water bath, and the contents are water-washed until no chlorideion remains, then dried at 110 C., comminuted and winnowed to obtain thespecimen. Of these 6 specimens, the tests were conducted on (A) thosedemonstrating the greatest decolorizing property when a lubricating oilwas decolorized at 250 C and (B) those demonstrating the greatestdecolorizing property when soybean oil was decolorized at 1 10 C.

2. Specimens Nos. 9-15.

a C. water bath. After the passage of each hour, one of the beakers istaken out by turns from the hot water bath and the contents arewater-washed until no sulfate ion remains, followed by drying at C.,comminution and winnowing to obtain the specimens. Of these specimens,the tests were conducted on (A) those demonstrating greatestdecolorizing property when a lubricant was decolorized at 250 C. and (B)those demonstrating the greatest decolorizing property when soybean oilwas decolorized at 110 C. These (A) and (B) specimens are indicated intable I as acid-treated specimens (A) and (B), respectively.

3. Specific surface area.

The specific surface area of the several specimens were determined bythe so-called BET method which is based on the adsorption of nitrogengas. For details of this method, reference shall be made to thefollowing literature:

S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc., 60,309 1938)perty. 1. Preparation of the solution of the color development coloringmatter.

As the primary color development coloring matter, crystal violetlactone, a triphenylmethane coloring matter, is used. 0.5 Gram of thiscoloring matter is dissolved in 99.5 grams of g.p. benzene. The chemicalnomenclature and structural formula of crystal violet lactone are asfollows:

Crystal violet lactone [3,3-bis(p-dimethylaminophenyl)-6-dimethylphthalide] 2. Color development conditions.

Two grams of the hereinbefore described dried specimen are weighed intoa watch glass 8 cm. in diameter and spread out thinly, after which 4 ml.of the aforesaid crystal violet lactone benzene solution are droppedonto the specimen in such a manner that the whole of the latter becomescompletely wet. This is followed by mixing the whole with a spatula toachieve a homogeneous mixture and allowing the mixture to stand for 1hour in a room temperature of l5-20 C. into which direct rays ofsunlight do not shine. Thus is obtained the specimen for measuring thecolor development. In the case where the oil absorption of the specimenis great and complete wetting is not had by 4 ml. of the solution, thesame operation is carried out after first having added the necessaryamount of benzene to the coloring matter solution.

3. Method of measuring the degree of color development.

The degree of color development of the specimen whose color has beendeveloped under the hereinbefore indicated conditions is measured usinga spectrophotomer. The specimen is packed in a quartz cell having adiameter of 21 mm. and a height of [0 mm. and with the width of the slit1 mm. the refiectances at wavelengths 390, 550 and 590 mp. are measured.The reflectance is indicated in percent using as the basis 100 percentfor the reflectance of an alpha-alumina shaped article.

4. Method of indicating the primary color development property.

When the reflectances at 390, 550 and 590 mu are respectively designatedR R and R the primary color development property, K,, is defined by thefollowing equation:

Thus, it can be said that the larger the value of K,, the better theprimary color development property. According to our experiments, therewas a very good agreement between the determinations of the colordevelopment property as expressed by the value of K, and the resultsobtained by determinations by means of the naked eye.

5. Method of measuring the secondary color development property.

The method of measuring and indicating this property, as previouslydescribed herein, is used.

6. Decolorizing property.

Fifty grams of unrefined soybean oil are weighed into a hard glassISO-ml. test tube, to which is then added 1 gram of the specimen. Thetest tube is then immersed in an oil bath heated at 1 10 C. and thecontents are stirred vigorously for 20 minutes. The specimen is thenfiltered with a filter paper, after which the clarified oil is placed ina 20-mrn. cell. White light is directed against this cell and the lighttransmittance is measured with a photoelectric colorimeter. The lighttransmittance is indicated in percent or the basis of percent for thelight transmittance of distilled water. The decolorizing property of oilwhen its light transmittance is T percent is defined by the followingequation:

where 56 percent is the light transmittance of the unrefined soybean oilused in the present experiment. 7. Criterion of suitability.

Those in which the secondary color development property is above 1.40and the specific surface area is above 180 mflg. are considered as beingacceptable.

The following facts can be comprehended from the results of table I.Namely, l. the natural clay minerals other than the dioctahedralmontrnorillorite clay minerals as used in the present invention are allinferior in their primary and secondary color development properties,the only exception being attapulgite (see specimen Nos. 2, 3 and 4 intable I). And even though their specific surface area is increased by anacid treatment, there is not much improvement of their primary colordevelopment property (see specimen No. 3 in table I).

2. While attapulgite demonstrates considerably good primary andsecondary color development properties in its as-obtained natural state,its specific surface area is not increased to as much as 180 m. /g. eventhough it is subjected to the acid treatment, and its primary colordevelopment property is also scarcely improved (see specimen No. l intable I).

3. On the other hand, a certain class of the dioctahedralmontmorillonite clays demonstrates good secondary color developmentproperty, which when acid treated increases its specific surface areagreatly to also demonstrate concomitantly a marked improvement in itsprimary color development property as well (see specimen Nos. 7, 9, l l,12 and I4 in table I).

4. However, there are those among the dioctahedral montmorillonite clayminerals which are unsuitable for use as the color former of the presentinvention because they are inferior in their secondary color developmentproperty although they excel in their specific surface area, primarycolor development property and decolorizing property (see specimen Nos.l0, l3 and 15 5. Further, there are those among the dioctahedralmontmorillonite clay minerals which excel in their secondary colordevelopment property as in the case with attapulgite but whose specificsurface area does not increase by means of the acid treatment and hencewhose primary color development property is not improved (see specimenNo. 8 in table I).

From the foregoing results it can be comprehended that the followinggeneral principles hold in the case of the dioctahedral montmorilloniteclay minerals.

a. The secondary color development property being an inherent propertyof the material clay itself cannot essentially be improved though someimprovement can be had by the acid treatment.

b. The primary color development property increases in proportion to anincrease in the specific surface area by means of the acid treatment,the desirable primary color development property (K,=2.60) beingattained when the specific surface area reaches or exceeds 180 m./g.

c. The decolorizing property is not related at all to the secondarycolor development property.

Hence, according to the present invention, a dioctahedralmontrnorillonite clay mineral having the highest possible K,

value, at least above 1.40, and preferably above 1.60, it first chosen.This is then subjected to an acid treatment so as to increase itsspecific surface area to above 180 mP/g. while ensuring that its K valuedoes not fall to below 1.40, and

preferably 1.60. Thus it becomes possible to produce the color 5 formerhaving basically satisfactory primary and secondary color developmentproperties. And, hence, as the aforesaid dioctahedral montmorilloniteclay minerals can be mentioned the natural clay minerals such, forexample, as the so-called bentonite, subbentonite, fullers earth,Florida earth and Japanese acid clay. However, it goes without sayingthat the dioctahedral montmorillonite clay minerals, as used herein, arenot limited to only those which have been illustrated.

On the other hand, as the acid to be used in the acid treatment that iscarried out in the invention process, any whether inorganic or organicmay be used which is able to increase the specific surface area of theaforesaid montmorillonite clay minerals to above I80 m. /g. However, theinorganic acids are generally to be preferred over the organic acids forreasons of cost and ease of handling, and of the inorganic acidssulfuric and hydrochloric acids are particularly convenient.

Further, no particularly strict conditions are involved in the acidtreatment. If an acid of dilute concentration is used, either thetreatment time becomes longer or the quantity of the acid requiredbecomes greater, whereas if the concentration is high, either thetreatment time becomes shorter or the quantity of the acid requiredbecomes less in correspondence to the increase in concentration. Again,if the treatment temperature becomes higher, the treatment time iscorrespondingly shortened. Hence, the acid concentration may be any inthe range of the order of 180, percent, but from the standpoint ofconvenience in handling practice the acid treatment is preferablycarried out at a concentration of the order of 1545 percent and atemperature ranging between 50 and l05 C. in short, in this invention,the acid treatment of the aforesaid dioctahedral montmorillonite claymineral until its specific surface area becomes at least 180 mF/g. isthe sole basically important conditions.

However, one thing which must be cautioned against the actually carryingout the acid treatment is that there are instances in which thesecondary color development property makes a marked decline when theacid treatment proceeds to an excessive degree. For this reason, it ispreferred that the acid treatment conditions be so controlled that thespecific surface area of the clay after treatment comes within the rangebetween 180 mF/g. and 350 mF/g.

Next, in table ll will be shown the changes in the specific surface areaand the primary and secondary color development properties ofacid-treated clay depending upon the it was also found by ourinvestigations that the particle size of the color former to be used inthe present invention was also a very important factor. That is to say,when the particle size of the color fonner becomes too large, there isan increase in the smudging phenomenon, whereas if there is an increasein those particles which are too small, the particles after having beenapplied to paper tend to exfoliate. Moreover, if considered from thestandpoint of the color development effect, a greater color developmenteffect is had when the particles of the color former is smaller in bothcases of the primary and secondary color development properties.

As a result of extensive researches for the reasons for these phenomena,we found that by making the particle size of the color former such thatthe particles of a diameter 10 microns or less were present in an amountof at least 75 percent by weight, and preferably at least 85 percent byweight of the total particles and those of a diameter one micron or lesswere present in an amount not exceeding percent by weight, andpreferably not exceeding 35 percent by weight of the total particles,the color development effect was greatly enhanced to yield an excellentcolor former for pressure sensitive recording paper use in whichmoreover the undesirable tendency to smudging and exfoliation of theapplied particles was less.

The relationship between the particle diameter of the color former andits primary and secondary color development properties and therelationship between the particle diameter and smudging are shown intable lll, below. It became apparent from these results that the colorformer should be preferably one containing at least 75 percent by weightof particles of a diameter 10 microns or less for achieving the colordevelopment efi'ect and prevention of the smudging phenomenon.

The specimens submitted to the experiment whose results have beenpresented in table [II were those prepared by adjusting the particlesize by winnowing of the acid-treated specimen (B). (No. 12) of thepreviously given table I.

TABLE III Content of Color development particles 10 propertySoil-resistant or less (wt. property Specimen N 0. percent) PrimarySecondary (percent) Method of measuring the soil-resistant property. I

One hundred grams of the color former are suspended in 250 ml. of water,to which are then added 10 grams of starch.

degree of acid treatment given.

TABLE 11 Japanese acid clay A Japanese acid clay B Specific PrimarySecondary Specific Primary Secondary Sulfuric acid surface color decolordesurface color decolor detreatment time area veloprnent velopment areavelopmeni; velopment (hr.) (ml/g.) property property (ml/g.) propertyproperty 0 (untreated clay) 76 2. 45 2.13 62 2.36 1. 62 0.5 102 2. 2. 15103 2. 42 1. 68 1 180 260 2. 15 142 2. 5O 1. 75 2 235 2. 99 2. 17 163 2.84 1. 70 3 265 3. 28 2. 24 187 3. 09 1. 4 295 3. 39 2. 31 230 3. 21 1.50 5 307 3. 47 2. 24 242 3. 3O 1. 38 6 325 3. 51 2. 15 248 3. 38 1.308..- 346 3.60 I. 88 263 3. 42 1. 28 370 3. 68 1. 65 271 3. 45 1. 25 3843. 74 1. 60 280 3. 48 1. 20

ra s showninfable ll, above, thedioctahedral mSBESrii-" lonite clayminerals as used in the present invention are increased in theirspecific surface area by means of the acid treatment, and it can be seenthat the primary color develop. ment property becomes good when thespecific surface area becomes greater than 180 mF/g. When the specificsurface area increases in this manner, the capacity to absorb and adsorboil increases at the same time. Hence, the acid treatment is necessaryand indispensable in the present invention.

This suspension is applied to paper of fine quality in an amount suchthat 7 grams of the color former are adhered per square meter of thepaper, after which the paper is dried to thus obtain a coated frontsheet. A coated back sheet is superposed on this coated front sheet, anda stainless steel cylinder 5 cm. in diameter and weighing 4 kg. is thenplaced on top of the superposed sheets and gently pulled across thesheets. The soiling of the coated front sheet thus resulting is measuredfor its reflectance of light of a wavelength 430 mp. using aspectrophotometer. The soil-resistant following equation:

property is defined by the Soil-resistant property That is to say, thegreater the numerical value of the soil-resistant property (percent),the less is the soiling.

It is thus apparent from the results given in table III that a markeddecrease in the smudging phenomenon takes place when the content ofparticles of a diameter microns or less is at least 75 percent byweight, and preferably at least 85 percent by weight of the totalparticles. It is also seen that the primary and secondary colordevelopment effects are also superior with this particle size.

A relationship between the particle diameter of the color former and theexfoliation property of the coated front sheet, such as shown in tableIV, below, is observed. In this experiment the 1.G.T. (lnstrturt voorGrafische Techniek) test was employed for investigating the exfoliationproperty. In this test the tendency to exfoliation is less as the numberbecomes greater. The specimens used are those whose particle diameterhas been adjusted as in table IV by winnowing the acidtreated specimen(A) (No. 12) of table 1.

The foregoing l.G.T. test was conducted on the specimen prepared in thefollowing manner in accordance with the below-described measurementmethod. One hundred grams of the color former are suspended in 250 ml.of water, to which are then added 10 grams of starch. This suspension isapplied to high quality paper such that the adhesion of the color formerto the paper amounts to 7 grams per square meter, followed by drying thepaper to obtain the coated front sheet. This sheet is submitted to thel.G.T. test and the rate at which the exfoliation of the color formertakes place is measured. It can be seen that when the content ofparticles one micron or less in size exceeds 45 percent by weight theresults of the l.G.T. test suddenly become worse.

It can be understood from the foregoing results that according to thepresent invention it is preferred from the standpoint of the colordevelopment efi'ect obtained and smudging that of the total particles atleast 75 percent by weight, and preferably 85 percent by weight, arethose whose particle size is 10 microns or less; and moreover that fromthe standpoint of the exfoliation property of the color former it ispreferred that of the total particles not more than 45 percent byweight, and preferably not more than 35 percent by weight, are those onemicron or less.

This exfoliation property is a considerably important matter in makinggood quality transfer sheets, because if the exfoliation of the colorformer is great a large amount of paste must be used for preventingthis, with the consequence that a marked decline in the colordevelopment effect takes place.

Thus, according to the present invention, success was achieved in theproduction of a color former for pressure sensitive recording paper by aprocedure comprising choosing from among the dioctahedralmontmorillonite clay minerals one whose secondary color developmentproperty, K value, relative to benzoyl leucomethylene blue as measuredin accordance with the hereinbefore described measurement method is atleast 1.40, and preferably at least 1.60, subjecting this chosenmontmorillonite clay mineral to an acid treatment to increase itsspecific surface area to at least 180 m. /g., and preferably to a valuein the range between 180 m. /g. and 350 mF/g. and moreover ensuring thatthe foregoing secondary color development property, K value does notbecome less than 1.40, and preferably not less than 1.60, followed bywater-washing and drying, and thereafter either comminuting orclassifying the foregoing clay mineral to render it into particle sizein which of the total particles at least 75 percent by weight are thosehaving a particle diameter 10 microns or less and moreover not more than45 percent by weight of the total particles are those one micron or lessin diameter.

In producing the invention color former, the procedure described aboveneed not necessarily be followed however, it being also possible toproduce it by the following method.

That is to say, the invention color former cal also be produced bymixing (A) a dioctahedral montmorillonite clay mineral which has beenacid treated until its specific surface area is at least 180 m."/g., andpreferably at least 220 m./g., with (B) a dioctahedral montmorilloniteclay mineral or an acid-treated product thereof whose secondary colordevelopment property, K has a value of at least 1.40, and preferably atleast 1.80 to obtain as a whole a specific surface area of at least 180mF/g. and a value for said K of at least 1.40, and comminuting orclassifying said clay minerals (A) and (B) either before or after theirmixture either separately or at the same time to render the mixture intoparticle sizes in which at least 75 percent by weight of the totalparticles are particles having a diameter 10 microns or less andmoreover not more than 45 percent by weight of the total particles arethose one micron or less in diameter.

When the hereinabove described method of the present invention isfollowed, the aforesaid clay mineral (A) need not necessarily be onewhose K value is at least 1.40. And on the other hand, the aforesaidclay mineral (B) or its acid-treated product need not necessarily be onewhose specific surface area is at least 180 m.'*/g.

The invention color former prepared as hereinbefore described can beapplied to paper using the natural or artificial pastes such, forexample, as starch, casein, tragacanth gum, CMC, synthetic latex havinga bonding property, such as styrene butadien latex andbutadiene-acrylonitrille resin latex and polyvinyl alcohol to thus makethe coated front sheet of pressure sensitive recording paper. Thus agood quality coated front sheet is obtained whose primary and secondarycolor development effects during copying are exceedingly good andmoreover in which smudging is held to a minimum.

Further, for improving the color development effect still further or forincreasing the amount of color former added, it is possible to suitablyadd such additives as other natural clay minerals or synthetic inorganicsubstances, e.g. calcium carbonate, silica, silicate; organic orinorganic pigments, e.g. ultramarine, persian blue, chrome yellow, ironoxide, lndanthrene, Rhodamine and Methyl violet; dyestuffs such asfluorescent bleaching agent, e.g. diaminostilbene and benzoimidazole;oxidants, e.g. chloroanyl, persulfates, dichromates, perhydrochlorides,perrnanganates, cupric salts, ferric salts, iodine, potassiumferrocyanide and organic acid peroxides; reducing agents, e.g., calciumsulfide and solid organic amines; solid acids, e.g. alumina, siloca,titania, zinc oxide, zinc chloride, titanium phosphate and zirconiumphosphate; and alkaline substances such as sodium silicate, sodiumpyrophosphate and alkaline earth metal hydroxides, e.g. slaked lime. Itis to be understood that these instances wherein additives have beenadded are also comprehended by the present invention.

EXAMPLES l-5 The material clays indicated in table V were chosen, whichwere each dried, comminuted and winnowed to prepare them into specimensin which percent of the total weight were particles whose diameter was10 microns or less and 30 percent of the total weight were those whosediameter was one micron or less.

EXAMPLE 6 Sixty-five percent by weight of the Japanese acid clayspecimen B of table ll which had been acid treated for 12 hours and 35percent by weight of specimen No. 8 of table I were mixed and thencomminuted. This comminuted mixture When the test for secondary colordevelopment property, as fully described herein, was conducted on theforegoing specimens, the K, values shown in table V were obtained.

TABLE V was then winnowed and a specimen having the following partif ggy cle size was prepared; i.e., 92 percent by weight of particles g;whose diameter was l0 microns or less and 23 percent by Specimenproperty, No Materialclay and locale of its production K, 10 weight ofparticles whose d ameter was one micron or less. When the properties ofthis specimen were tested by the 1 3 1 55 3 acid i Nugata -1 A methodsfully described in connection with table I, the follow- 2 B 53 ingresults were obtained. 3.. .do C 1.31 Japanese acid clay-Tsuruoka,Yamagata A 2.15

Pref, Japan. 15 Specific surface area I97 m./g. 5 ..do B 1.39 K, 3.05 d0C 1. 35 Kg 5g apanese acid clay-Shibata, Niigara. Pref, A 1. 29

apan. g "g8 B 53 The invention color former was thus obtained. 16: I'iixitoriii 'isn swam elitism; 11;: I 1: 70 11 g p 1. so 20 EXAMPLE7Sub-bentonite, M ssissippi, U.S.A

Specimen l (specimen No. l of table I) and specimen ll (Japanese acidclay specimen 8 of table II which had been acid treated for 12 hours)were each winnowed and specimens The Value of K in the Specimens L 4, 10and l 1 in having the following particle sizes were prepared. table Vwere above 1.40. Hence, the acid treatment of these was carried outunder the following conditions.

Specimen I0 4. or less I p. or less Fifty grams on a dry basis of eachof the foregoing five classes of clays were weighed into 500-ml. conicalbeakers and I 90 M 2| wt I their acid treatment was carried out underthe conditions in- 93 w 4 m dicated in table lV, followed bywater-washing, drying at 1 10 C., comminuting and winnowing to preparethe specimens. The particle size of each specimen was in all cases 90percent of particles l0 microns or less in diameter and 25 percent ofthose one micron or less.

Next, 20 percent by weight of specimen 1 and 80 percent by weight ofspecimen II were well mixed, after which the proper- 5 ties of mixturewas tested by methods which were fully described in connection withtable I, with the following results: 7 7 TABLE VI Specific surface area3 l0 m."/g.

Acid treatment conditions Concen- 'lemtration The invention color formerwas thus obtained. peigature (wt. Time Class of acid 0.) percent) (hr.)EXAMPLE 8 Sulfuric acid. 86 34 2 .do s5 34 3 The acid-treated specimen(B) (specimen No. l l of table I) gf gg g g8 is 2 was winnowed and aspecimen having the following particle Acetic acidil. 85 27 6 size wasprepared.

MWH w i U "*7 2 l0 microns or less 02 wt. k l micron or less 26 wt. I

To 50 grams of this specimen was then added 0.25 gram of e.p. sodiumperborate as an oxidant followed by thorough The properties of theseveral specimens obtained by the foregoing acid treatment are shown intable Vll, below.

TABLE VII Color Specific development Decoior- Soil- Specimen surfacevalue izing resistant number in area. ability property I.G.'I. Table V(m./g.) K1 K2 (Percent) (percent) (cam/sec.)

As apparent from the foregoing description, when those having a K, valueof at least 1.40 are chosen from the various classes of the dioctahedralmontmorillonite clay minerals and these chosen minerals are subjected toan acid treatment 70 mixing. When this mixture was tested for itsproperties in accordance with the testing methods fully described inconnection with table I, the following results were obtained.

Specific surface area 280 m./g.

Thus was obtained the color former according to the 75 presentinvention.

. 5 EXAMPLE 9 The acid-treated specimen (B) (specimen No. 12 of table I)was winnowed and a specimen having the following particle sizes wasprepared.

I0 microns or less 1 micron or less Next, to I00 grams of this specimenwere added 4 grams of slaked lime as an alkaline substance followed bythorough mixing. When this specimen was tested for its properties by thetest methods fully described in connection with table I, the resultsobtained were as follows:

Specific surface area 340 m.=/g. K, 3.15 K, 1.89

wherein R and R are reflectances of light having wavelengths 430 mu and550 mu, respectively, when said minerals are developed by benzoylleucomethylene blue. 2. A process for producing the color former forpressure sensitive recording paper, said process comprising treating adioctahedral montmorillonite clay mineral having a secondary colordevelopment property, K of at least 1.40, the value of K beingrepresented by the formula wherein R and R are reflectances of lighthaving wavelengths 430 my and 550 m,u., respectively, when said mineralis developed by benzoyl leucomethylene blue, with an acid to increasethe specific surface area of said mineral to at least 180 mF/g. whileensuring that said secondary color development property, K value doesnot decline .those of diameter 10 microns below 1.40; washing theacid-treated mineral with water and drying the acid-treated mineral; andthereafter comminuting and classifying the mineral to render it intoparticle sizes in which of the total particles at least 75 percent byweight are those of diameter 10 microns or less and of the totalparticles not more than 45 percent by weight are those of diameter onemicron or less.

3. A process for producing a color former for pressure sensitiverecording paper, said process comprising mixing:

A. a dioctahedral montmorillonite clay mineral which has been acidtreated until its specific surface area has been increased to 180 m, lg.with B. dioctahedral montmon'llonite clay minerals having a secondarycolor development property, K; of at least 1.40, the value of K beingrepresented by the formula wherein MEIER are wavelengths 430 mp. and 550my respectively, when said member is developed by benzoyl leucomethyleneblue, comminuting and classifying said mixture of minerals to rendersaid mixture of minerals as a whole into one whose specific surface areais at least 180 m. /g., whose secondary color development property, K,is a value at least 1.40, and in which of the total particles at least75 percent by weight are or less and of the total particles not morethan 45 percent by weight are those of diameter one micron or less.

4. The process of claim 2 wherein the treatment with acid is conductedat a temperature of 50-l05 C. with an acid concentration of lpercent.

5. A color former for pressure sensitive recording paper which comprisesa major portion of acid-treated dioctahedral montmorillonite clayminerals and a minor portion of natural dioctahedral montmorilloniteclay minerals, said mixture as a whole having a specific surface area ofat least m. /g., of the total particles at least 75 percent by weight ofparticle diameter 10 microns or less, and of the total particles notmore than 45 percent by weight being those of diameter one micron orless, said mixture as a whole having a secondary color developmentproperty, K at least 1.40, the value of K being represented by theformula K2=%+i 1mw wherein R and R are reflectances of light havingwavelengths 430 my. and 555 mu, respectively, when said mixtures aredeveloped by benzoyl leucomethylene blue.

iw: a r

2. A process for producing the color former for pressure sensitiverecording paper, said process comprising treating a dioctahedralmontmorillonite clay mineral having a secondary color developmentproperty, K2 of at least 1.40, the value of K2 being represented by theformula wherein R430 and R550 are reflectances of light havingwavelengths 430 m Mu and 550 m Mu , respectively, when said mineral isdeveloped by benzoyl leuco methylene blue, with an acid to increase thespecific surface area of said mineral to at least 180 m.2/g. whileensuring that said secondary color development property, K2 value doesnot decline below 1.40; washing the acid-treated mineral with water anddrying the acid-treated mineral; and thereafter comminuting andclassifying the mineral to render it into particle sizes in which of thetotal particles at least 75 percent by weight are those of diameter 10microns or less and of the total particles not more than 45 percent byweight are those of diameter one micron or less.
 3. A process forproducing a color former for pressure sensitive recording paper, saidprocess comprising mixing A. a dioctahedral montmorillonite clay mineralwhich has been acid treated until its specific surface area has beenincreased to 180 m.2/g. with B. dioctahedral montmorillonite clayminerals having a secondary color development property, K2 of at least1.40, the value of K2 being represented by the formula wherein R430 andR550 are wavelengths 430 m Mu and 550 m Mu , respectively, when saidmember is developed by benzoyl leucomethylene blue, comminuting andclassifying said mixture of minerals to render said mixture of mineralsas a whole into one whose specific surface area is at least 180 m.2/g.,whose secondary color developmenT property, K2 is a value at least 1.40,and in which of the total particles at least 75 percent by weight arethose of diameter 10 microns or less and of the total particles not morethan 45 percent by weight are those of diameter one micron or less. 4.The process of claim 2 wherein the treatment with acid is conducted at atemperature of 50*-105* C. with an acid concentration of 1-80 percent.5. A color former for pressure sensitive recording paper which comprisesa major portion of acid-treated dioctahedral montmorillonite clayminerals and a minor portion of natural dioctahedral montmorilloniteclay minerals, said mixture as a whole having a specific surface area ofat least 180 m.2/g., of the total particles at least 75 percent byweight of particle diameter 10 microns or less, and of the totalparticles not more than 45 percent by weight being those of diameter onemicron or less, said mixture as a whole having a secondary colordevelopment property, K2 at least 1.40, the value of K2 beingrepresented by the formula wherein R430 and R550 are reflectances oflight having wavelengths 430 m Mu and 555 m Mu , respectively, when saidmixtures are developed by benzoyl leucomethylene blue.